1. Field of the Invention
The invention relates to an electronic ballast for supplying a load, for example a lamp, in particular a gas discharge lamp, with an a.c. voltage the frequency of which is higher than the mains frequency, having a rectifier for connection to the a.c. mains, an inverter downstream of the rectifier, which inverter generates the higher frequency a.c. voltage for the load, and a harmonics filter arranged between the rectifier and the inverter, the output voltage of which harmonics filter is controlled.
2. Description of the Related Art
When operated at mains frequency, electronic gas discharge lamps have an unsatisfactory light yield and tend to flicker. For this reason, they are usually operated with electronic ballasts which work with mains voltage and generate therefrom a higher frequency operating voltage for the gas discharge lamps, the frequency of which lies in the region from 20 to 50 kHz. The electronic ballasts comprise a rectifier which rectifies the mains voltage, and an inverter which generates the higher frequency operating voltage from the rectified mains voltage. The light yield of gas discharge lamps can be appreciably increased, and flickering can be avoided, by the use of electronic ballasts but at the same time a problem appears in that as a result of the switching processes in the rectifier there occur harmonics which cause unacceptable disturbances when fed back into the mains. To avoid this, a passive filter can be connected between the rectifier and the mains, but this filter must comprise a relatively large and expensive choke. This choke can be avoided if, in place of the passive filter, an electronic harmonics filter is employed, connected between the rectifier and the inverter. The electronic harmonics filter manifests itself in that it removes pulse-like current from the rectifier, whereby the envelope curve of the current pulses is approximated to the sinusoidal half-wave form of the mains frequency.
One such harmonics filter is described, for example, in EP 268 043 A1 or in the book by C. H. Sturm/E. Klein "Betriebsgerate und Schaltungen fur Lampen", Verlag SIEMENS AG, 6. Auflage, 1992 ("Operating devices and circuits for lamps", published by SIEMENS AG, 6th edition, 1992) pages 127/128. The electronic harmonics filter comprises a clocked switching controller which may be, for example, an step-up transformer (booster) or an inverting transformer. The principle of such a clocked switching controller is described in the book by U. Tietze, Ch. Schenk "Halbleiter-Schaltungstechnik" Springer Verlag, 9. Auflage ("Semiconductor circuitry techniques", published by Springer, 9th edition) pages 563 to 570.
With known harmonics filters the output voltage of the switching controller is constantly monitored and compared with a fixed desired-value voltage. For the switch of the switching controller, the controller generates from the control difference a control signal of variable switching frequency. By varying the switching frequency, the output voltage is held constant.
In particular for gas discharge lamps there is additionally the need to alter the operating voltage for the purpose of setting the brightness. Normally this is achieved for lamps by connecting a dimmer in the mains feed. The dimmer can, for example, comprise a phase angle control circuit such as is described in the book by C. A. Sturm/E. Klein on page 141 and in the book by Klaus Beut "Bauelemente der Elektronik, Elektronik 2", Vogel Verlag, 1975 ("Electronic Components, Electronics 2") page 242. The dimmer generates from the mains a.c. voltage pulses of alternating polarity with variable width. By means of a subsequent rectification a d.c. voltage can be generated from these pulses the level of which d.c. voltage is alterable by altering the degree of dimming (phase angle control). In this way, the voltage supplied to the inverter is varied when an electronic ballast without harmonics filter is employed, with the consequence that the operating voltage for the lamp, and thus the brightness of the lamp, is also altered.
The situation is different when the ballast includes an electronic harmonics filter. The latter has, as indicated above, the tendency to maintain the output voltage constant. This means, that, for example, a dimming of the lamp is not possible with the use of a simple dimmer inserted in the mains feed.